The present invention generally relates to a heat exchanger of the plate-fin type for use in oil coolers, condensers, evaporators, etc., and more particularly to a heat exchanger of the plate-fin type having fins of one metal brazed to a plate made of a different metal.
The plate-fin type heat exchanger has a large area of heat transfer per unit area and a high coefficient of heat transfer, and therefore, has an advantage in that it is compact and relatively easy to manufacture in comparison with other types. Also, the plate-fin type heat exchanger has a wide range of design variations available, such as the fin pitch, the fin height and the fin shape. These variations may be chosen to provide a heat exchanger suitable for the nature and the purpose of the fluid flowing through each passage.
Referring to FIG. 1, a conventional plate-fin heat exchanger 100 may be made by brazing a first set of fins 110 to a one side of a metal plate member 120. A second set of fins 130 may be brazed to the other side of metal plate member 120. Fins 110 and 130 may be configured to provide a counter-current flow (not shown) or a cross-current flow between the first set of fins 110 and the second set of fins 130. Flat plates 140, 150 may be brazed to fins 130 and 110, respectively, to provide cross or counter-current-flowing fluid passages A and B.
Titanium alloys have been used to make these conventional plate-fin heat exchangers due to the light weight and high heat capacity of the material. These conventional heat exchangers, however, can have a problem with fin distortion during the brazing step in the manufacturing process. Titanium fins that are tall, thin and widely spaced cannot support the loads imposed on them by the fixture and heat exchanger sections on top of them. This often leads to crushing of the fins and unbrazed areas within the heat exchanger. Furthermore, tall fins are difficult to form out of titanium due to cracking and the presence of burrs on offset fins.
Titanium alloys also have a relatively low thermal conductivity and that is especially true of the high strength alloys. Commercially pure (CP) titanium has the highest conductivity but it also has the lowest strength which may not be suitable for a heat exchanger component. Higher conductivity fin materials of a different base metal can increase the performance of a heat exchanger.
U.S. Pat. No. 4,725,509 describes a braze filler and method of brazing where the filler is a titanium-copper-nickel alloy and the parent metal is titanium, nickel, cobalt or an iron-based alloy. The braze filler used in the brazing method of the patent requires a temperature of at least 1700° F. (column 3, lines 1–5). Brazing at this temperature, however, may cause the filler to erode some titanium base metals, and thus, may not prove useful as a brazing method for dissimilar metals when one of the metals is a titanium alloy.
U.S. Pat. No. 6,168,069 describes a method of joining cylindrical shapes of titanium to stainless steel using a silver-copper-palladium braze. Silver, however, is not desirable in contact with titanium due to a potential for embrittlement of the titanium when in contact with liquid or solid silver. Moreover, the brazing method of this patent requires the joint be configured so that it is in compression after brazing. Finally, the brazing method of this patent requires that the joint be fitted together and the filler metal be placed outside the joint prior to brazing. Placing brazing filler outside of the joint may not be practical when creating the thousands of fin-to-tube sheet joints as is present inside a plate-fin type heat exchanger.
As can be seen, there is a need for an improved plate-fin titanium based heat exchanger that uses materials resistant to crushing at the brazing temperature while being simple to design and manufacture.